To determine the risk of severe Haemophilus influenzae illness among household contacts of patients with H. influenzae meningitis, we studied prospective data obtained in 19 states from January 1, 1977, to June 30, 1978. H. influenzae meningitis was reported in 1403 patients, and 1147 (82 per cent) of the exposed families were investigated for the occurrence of H. influenzae disease within 30 days after its onset in the index patient. During this interval, nine of 1687 household contacts (0.5 per cent) under the age of six years had systemic disease confirmed to be caused by H. influenzae Type b. The risk in children less than one year of age was 6 per cent, and the risk in those less than four years of age was 2.1 per cent. None of 2624 contacts above the age of five was affected. In the 30 days after onset of meningitis, the risk of this infection alone, aside from other types of serious H. influenzae disease, is 585 times greater in household contacts than the age-adjusted risk in the general population. The risk of H. influenzae disease in household contacts under six years of age is similar to the risk of secondary meningococcal disease in all household contacts--indicating a need for effective antimicrobial prophylaxis.